(1) Field
The disclosed methods and systems relate generally to winding optical fiber, and in particular to systems and methods to wind the optical fiber based on an eigenfrequency and/or scale factor.
(2) Description of Relevant Art
Fiber optic gyroscopes (FOGs) have evolved in recent decades from laboratory demonstration to practical use. FOGs can be used in many applications including navigational systems and stabilization platforms. The FOG operational principle for sensing inertial rotation is optical rather than mechanical. FOGs can use the well-known Sagnac effect to sense the relative increase in optical pathlength when a coil of optical fiber is rotated about its axis. Accordingly, consider a beam of light that is split into two beams that are directed in opposite directions around a loop and to a detector. In theory, the two beams will travel equal distances and arrive at the detector at the same time and in-phase. The Sagnac effect indicates that if the entire device or loop is rotating, the beam traveling in the direction of rotation will travel further than the beam traveling opposite to the rotation, and hence the beams will arrive at the detector at different times and out of phase, thereby producing measurable optical fringes. Accordingly, the length of the optical path and the diameter of the optical winding can determine the FOG accuracy.
Generally, FOGs can be constructed by winding a fiber optic cable around a generally circular spool with a number of turns holding the fiber optic cable in place. As indicated previously, light can be broadcast into both ends of the fiber optic cable and a detector or other sensor can record the light exiting the cable ends. Interference patterns between the exiting light waves can indicate changes in rotational motion of the FOG in the plane through the toroid formed by the fiber optic cable.
FOGs can be manufactured based on a particular frequency or propagation time, otherwise known as an eigenfrequency, that is based on the coil and other fiber parameters. Accordingly, the eigenfrequency can be understood to be inversely proportional to the propagation time through the fiber optic coil. Because the eigenfrequency, and also propagation time, can be effected by variations in fiber core diameter, coil length, and geometry, it can be difficult to wind a FOG coil for a specific and/or predetermined eigenfrequency (i.e., propagation time). Although the operational frequency of a FOG can approximately correspond to coil length, this relationship is not exact, and FOG winding techniques that contend with merely this single relationship can inspire FOG manufacturing methodologies that provide inconsistent and inaccurate results with respect to a desired eigenfrequency.
The disclosed methods and systems include winding optical fiber to an eigenfrequency and/or a scale factor. In one embodiment, the methods and systems can be used to tune a fiber optic gyroscope (FOG) to an eigenfrequency. In the FOG embodiment, a fiber optic coil assembly can include a normal end and a reverse end, a normal breakout point and a reverse breakout point, a normal lead and a reverse lead provided from the normal and reverse breakout points toward the normal and reverse fiber ends, and a normal and a reverse connection point for connecting a normal and a reverse splice to at least one of normal and reverse leads and normal and reverse ends. The normal and reverse splices can be secured by, for example, epoxy, to form a zipped section. The fiber optic coil can be wrapped between the normal breakout point and the reverse breakout point, around a spool, and such section can be known as an inner coil. The fiber optic coil from the normal and reverse breakout points, to the normal and reverse ends of the zipped section, can also be wrapped around the spool, to cause a reversal of wrapping direction of one of the reverse splice, the reverse connection point, and/or the reverse lead.
The normal and reverse splices and/or leads can be adjusted to provide a length that is based on the desired eigenfrequency. The length can be increased or decreased. Furthermore, a set of lengths can be established. For example, a first length can be provided between the reversal and the reverse breakout point, a second length between the reversal and the reverse connection point, a third length between the normal connection point and the normal breakout point, a fourth length between the normal breakout point and the reverse breakout point xe2x80x9cinner coilxe2x80x9d), a fifth length between the reverse end and the reverse connection point, and a sixth length between the normal end and the normal connection point. The lengths can be further categorized as one of xe2x80x9cplusxe2x80x9d or xe2x80x9cforwardxe2x80x9d lengths, or xe2x80x9cminusxe2x80x9d or xe2x80x9creversexe2x80x9d lengths, based on the direction of winding around the spool. In one embodiment, the first, third, and sixth lengths can be xe2x80x9cplusxe2x80x9d lengths (all same direction), while the second and fifth lengths can be xe2x80x9cminusxe2x80x9d lengths (all same direction, but opposite of the plus lengths). In some embodiments, the fourth length cannot be adjusted. Accordingly, based on an excess length, a measured eigenfrequency, the desired eigenfrequency, the fourth length, an outside diameter of the inner coil, and an effective mean diameter of the inner coil, a desired plus and minus length can be computed. One or more of the lengths contributing to the plus and minus lengths, namely lengths one, three, and six, and lengths two and five, respectively, can be adjusted to achieve the desired plus and minus lengths. Furthermore, the reversal can be adjusted.